Solid microlaser passively switched by a saturable absorber and its production process

ABSTRACT

The invention relates to a microlaser cavity ( 10 ) having: 
     a solid active medium ( 2 ) emitting at least in a wavelength range between 1.5 and 1.6 μm, and 
     a saturable absorber ( 4 ) of formula CaF 2 :Co 2+  or MgF 2 :Co 2+  or SrF 2 :Co 2+  or BaF 2 :Co 2+  or La 0.9 Mg 0.5-x Co x Al 11.433 O 19  or YalO 3 :Co 2+  (or YAl 5-2x Co x Si x O 3 YAl (1-2x)   Co   x   Si   x   O   3 ) or Y 3 Al 5-x-y Ga x Sc y O 12 :Co 2+  (or  -3 Al 5-x-y2z Ga x Sc y Co z Si z O 12 Y 3   Al   5-x-y-2z   Ga   x   Sc   y   Co   z   Si   z   O   12 ) or Y 3-x Lu x Al 5 O 12 :Co 2+  (or Y 3-x Lu x Al 5-2y Co y Si y O 3 ) or Sr 1-x Mg x La y Al 12-y O 12 :Co 2+  (or Sr 1-x Mg x-y Co y La z Al 12-z O 12 , with 0&lt;y&lt;x)  Sr 1-x   La   x   Mg   x   Al   12-x   O   19   :Co   2+  ( or Sr   1-x   La   x   Mg   x-y   Co   y   Al   12-x   O   19    , with 0&lt;y&lt;x ).

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a solid microlaser, a cavity for saidmicrolaser and a process for the production of said cavity.

One of the advantages of the microlaser is its structure in the form ofa stack of multilayers. The active laser medium is constituted by amaterial of limited thickness between 150 and 1000 μm and smalldimensions (a few mm²), on which are directly deposited dielectriccavity mirrors. This active medium can be pumped by a III-V laser diode,which is either directly hybridized on the microlaser, or coupled to thelatter by an optical fibre. The possibility of collective productionusing microelectronic means allows a mass production of such microlasersat very low cost.

Microlasers have numerous applications in fields as varied as the carindustry, the environment, scientific instrumentation and telemetry.

Known microlasers generally have a continuous emission of a few dozen mWpower. However, most of the aforementioned applications require peakpowers (instantaneous power) of a few kW delivered for 10⁻⁸ to 10⁻⁹seconds, with a mean power of a few dozen mW. In solid lasers, it ispossible to obtain such high peak powers by making them operate in thepulsed mode at frequencies between 10 and 10⁴ Hz. For this purpose useis made of well known switching methods, e.g. the Q-switch.

More specifically, the switching of a laser cavity consists of adding toit time-variable losses, which will prevent the laser effect for acertain time during which the pumping energy is stored in the excitedlevel of the gain material. These losses are suddenly reduced at precisemoments, thus feeing the stored energy in a very short time (giantpulse). Thus, a high peak power is obtained.

In the case of so-called active switching, the value of the losses isexternally controlled by the user (e.g. intracavity electrooptical oracoustooptical, rotary cavity mirror changing either the path of thebeam, or its polarization state). The storage time, the time of openingthe cavity and the repetition rate can be independently chosen. However,this requires adapted electronics and makes the laser system morecomplicated. An actively switched microlaser is described in EP-724 316.

In the case of so-called passive switching, variable losses areintroduced into the cavity in the form of a material known as asaturable absorber (S.A.), which is highly absorbant at the laserwavelength and has a low power density and which becomes virtuallytransparent when said density exceeds a certain threshold, which iscalled the S.A. saturation intensity.

A passively switched microlaser is described in EP-653 824.

The latter document more particularly describes a microlaser having:

a solid active medium, which can be constituted by a base materialchosen from among Y₃Al₅O₁₂, LaMgAl₁₁O₁₉, YVO₄, Y₂SiO₅, YLiF₄ or GdVO₄,doped with erbium (Er) or an erbium-ytterbium (Er—Yb) codoping),

a saturable absorber deposited by liquid phase epitaxy directly on thesolid active medium and constituted by a base material, identical tothat of the solid active medium, and doped with Er³⁺ ions.

This microlaser makes it possible to obtain an emission length ofapproximately 1.5 μm. This emission length has a particular interest,particularly in the field of optical telecommunications. The diversityof specific applications in this field makes it necessary to have othermicrolaser sources making it possible to emit at or close to thiswavelength.

The article by M. B. Camargo et al entitled “Co²⁺:YSGG saturableabsorber Q-switch for infrared erbium lasers”, published in OpticsLetters, vol. 20, No. 3, pp 339-341, 1995, describes; a laser passivelyswitched with the aid of a saturable absorber Co²⁺:Y₃Sc₂Ga₃O₁₂ and asaturable absorber Co²⁺:Y₃Al₅O₁₂. Thus, it is a saturable absorber basedon YAG or YSGG and doped with Co²⁺. However, for the reentry of a 2⁺charging ion, it is necessary to carry out a charge compensation of thesubstrate with a 4⁺ ion (charged four times positively), in order tomaintain the neutrality of the compound. This charge compensationproblem is made all the more difficult to solve in that the crystalsdescribed in the article by M. B. Carmago et al are firstly produced insolid form and are then cut up into sections. Moreover, the methoddescribed in this document for producing the crystal (Czochralskimethod) limits the concentration of dopants which it is possible tointroduce into the matrix, as a result of stability problems. Finally,this document provides no specific construction for an operation with ahigh repetition frequency.

DESCRIPTION OF THE INVENTION

In order to solve these problems, the invention relates to a monolithic,solid microlaser emitting at at least one wavelength in the infraredrange exceeding 1.5 μm and which supplies a pulsed beam by a passiveswitching process. Such a device will preferably operate at a highrepetition frequency. The microlaser is formed from at least twomaterials, namely a solid amplifier medium and a saturable absorbermedium, or which serves the purpose of a saturable absorber, i.e. aself-modulated loss modulator.

The invention relates to a microlaser cavity having a solid activemedium emitting at least in a wavelength range between 1.5 and 1.6 μmand a saturable absorber of formula CaF₂:Co²+ or MgF₂:Co²⁺ or SrF₂:Co²⁺or BaF₂:Co²⁺ or La_(0.9)Mg_(0.5-x)Co_(x)Al_(11.433)O₁₉ or YAlO₃:Co²⁺ (orYAl_(5-2x)Co_(x)Si_(x)O₃YAl_((1-2x)) Co _(x) Si _(x) O ₃) orY₃Al_(5-x-y)Ga_(x)Sc_(y)O₁₂:Co²⁺ (orY₃Al_(5-x-y-2z)Ga_(x)Sc_(y)CO_(z)Si_(z)O₁₂) or Y_(3-x)Lu_(x)Al₅O₁₂:Co²⁺(or Y_(3-x)Lu_(x)Al_(5-2y)Co_(y)Si_(y)O₃) orSr_(1-x)Mg_(x)La_(y)Al_(12-y)O₁₂:Co²⁺ (orSr_(1-x)Mg_(x-y)Co_(y)La_(z)Al_(12-z)O₁₂, with o<y 21 x) Sr_(1-x) La_(x) Mg _(x) Al _(12-x) O ₁₉ :Co ²⁺ (or Sr _(1-x) La _(x) Mg _(x-y) Co_(y) Al _(12-x) O ₁₉ , with 0<y<x).

All these saturable absorber compositions make it possible to obtain asaturable absorber element in thin film form, e.g. by molecular beamepitaxy, or by sol-gel deposition. The films have the advantage of beingstressed more easily than solid structures.

Moreover, the compositions CaF₂:Co²⁺, MgF₂:Co²⁺, SrF₂:Co²⁺, BaF₂:Co²⁺ orLa_(0.9)Mg_(0.5-x)Co_(x)Al_(11.433)O₁₉ do not require a chargecompensation, due to the introduction of cobalt as a dopant.Consequently, the saturable absorber can be a film with a thicknessbetween e.g. 1 and 200 μm (e.g. between 5 and 150 μm).

A microlens can be provided on one of the faces of the microlaser, whichmakes it possible to stabilize the cavity and helps to lower theoperating threshold of the microlaser.

The saturable absorber can be combined with the amplifier medium orsolid active medium by various processes:

by assembly with an optical adhesive or a resin bead,

by deposition in film form (e.g. a Co²⁺-doped sol-gel film, or aCo²⁺-doped, epitaxied film),

or by a mixture of the two processes involving firstly a deposition on asubstrate which is not the laser material, followed by the assembly ofsaid substrate with the amplifier medium, after which the film substratecan be removed, or it can already contain one of the mirrors of themicrolaser cavity.

Thus, the saturable absorber can be a material of different types:

a crystal doped with a saturable absorber ion (e.g. Co²⁺-doped LMA),

a monocrystalline film doped with a saturable absorber ion (e.g.LMA:Co²⁺, or CaF₂:Co²⁺ or MgF₂:Co²⁺ or SrF₂:Co²⁺ or BaF₂:Co²⁺), e.g.deposited by liquid phase epitaxy on the laser crystal, which is matrixor stress-matched,

a sol-gel film deposited on the laser material and doped with a Co²⁺saturable absorber ion.

The microlaser or microlaser cavity according to the invention canoperate at a high repetition frequency (≧100 or 200 Hz).

The invention also relates to a process for the production of a deviceas described hereinbefore.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

FIG. 1 Diagrammatically a microlaser according to the invention.

FIG. 2 Diagrammatically another microlaser structure according to theinvention.

FIGS. 3 & 4 Stages in the production of a microlaser according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A microlaser or microlaser cavity according to the invention is shown inFIG. 1 and has a solid active medium 2 and a saturable absorber 4. Thesetwo elements are placed between two mirrors 6, 8, which seal the lasercavity. Reference numeral 10 designates the complete cavity and 12 and14 respectively designate a pumping beam of the microlaser cavity and anemitted beam.

A microlaser according to the invention is combined with cavity pumpingmeans, which are not shown in the drawing.

The constituent material of the active medium 2 is e.g. doped witherbium (Er) for a laser emission of approximately 1.5 μm. This materialcould e.g. be chosen from among the following materials:

YAG (Y₃Al₅O₁₂), LMA (LaMgAl₁₁O₁₉), YSO (Y₂SiO₅), YSGG (Y₃Sc₂Ga₃O₁₂),GdVO₄, SYS (SrY₄(SiO₄)₃O), CAS (Ca₂Al₂SiO₇), etc. This choice will beconditioned by a certain number of criteria, but will also depend on theapplications envisaged. Among these criteria, reference can be made tothe following:

a high absorption coefficient at the pumping wavelength (e.g. III-Vlaser diode emitting at about 980 nm), for increasing the pumpingefficiency, whilst still maintaining a limited material thickness (<1mm);

a wide absorption band at the wavelength of the pumping beam, in orderto deal with the problem of wavelength stabilization of the laser diode,thereby simplifying the choice and electrical control of the laserpumping diode;

a large, stimulated, effective emission cross-section for obtaining highefficiency and high output powers;

a small emission band width to easily obtain a monofrequency laser, orconversely a broad emission band to achieve a frequency-tunable laseremission;

good thermomechanical properties in order to simplify the machining ofthe material and to limit prejudicial thermal effects by a goodevacuation of the heat produced by the absorption of the pump (thisexcess heat will depend on the energy efficiency of the laser);

a long life in the excited state for a high energy storage, or a shortlife for a rapid switching rate;

the possibility of finding the material in large sizes in order to beable to simultaneously collectively produce the largest number possibleof microlasers with one laser crystal.

In general, none of the known materials simultaneously satisfies allthese criteria. However, among the known materials, those which are moreparticularly adapted to the operation of the microlaser, particularly inthe case of a Nd³⁺ doping, are (with comparable life periods of a fewhundred microseconds):

YVO₄, which has a good coefficient, a wide absorption band a goodeffective cross-section, but a poor thermal conductivity, whilst it isonly obtained in small sizes and is also fragile;

YAG, whose absorption coefficient and effective, stimulated emissionsection are average and whose emission and absorption band widths aresmall, but is available in large sizes and with a good thermalconductivity;

LMA, which offers low effective cross-section and absorptioncoefficient, but wide absorption and emission bands, whilst being oflarge size, but poor thermal conductivity.

Another possible material is phosphate glass doped with erbium (dopingbetween 0.5 and 0.9% oxide) and ytterbium (between 15 and 20% oxide).

For an emission at 1.5 μm and higher, a choice will be made of activeions, e.g. from among erbium (Er) or chromium (Cr) or anerbium-ytterbium (Er+Yb) codoping or an erbium-ytterbium-cerium(Er+Yb+Ce) codoping. Compared with doping using erbium alone, Er+Ybcodoping makes it possible to absorb more pumping energy. The Yb ionthen absorbs the pumping beam and transfers the energy to the Er ion.

Erbium-doped materials have an emission band at a wavelength of around1.5 μm. This band is relatively wide and in glass reaches a width of 25nm. It can be more or less wide and more or less displaced as a functionof the matrix doped with the erbium ion. Thus, in YAG, the band iscentred at around 1.64 μm.

In a laser, the erbium ion has a precise emission wavelength, but whichvaries within said band. Thus, in a glass doped with Er—Yb wavelengthsfrom 1.53 to 1.58 μm have been observed. This is explained by the factthat the clear gain of the laser varies with its pumping power and itsmaximum is displaced.

The clear gain is obtained by subtracting the losses by absorption fromthe gain of the amplifier medium. The erbium ion gives three level lasersystems. This means not only that the laser medium has gain at thewavelength, but also has losses by absorption at said same wavelength.When the laser operates, there are simultaneously losses and gain on thepart of the laser medium. However, these two phenomena are of varyingamplitude as a function of the operating wavelength. They are alsodependent on parameters of the laser. Thus, the situation can arise thatfor a certain configuration (fixed by the pumping power, the length andconcentration of the laser material, the transmission of the mirrors andthe losses of the cavity), the clear gain is less great than in anotherconfiguration for a given wavelength. For another configuration, themaximum, clear gain wavelength will differ. In particular, it ispossible to vary said wavelength by varying the pumping power.

For further details of the thickness e of the active medium 2, referencecan be made to EP-653 824, which gives typical thicknesses of lasermedia for a monomodal operation, as well as the relationship between thethickness and the number of modes.

The saturable absorber can be chosen from among the following compounds:

CaF₂ doped with cobalt (CaF₂:Co²⁺), or compounds of the same family:MgF₂:Co²⁺, SrF₂:Co²⁺, BaF₂:Co²⁺.

Compounds from the LMA family (LaMgAl₁₁O₁₉) doped with cobalt, e.g. in aproportion greater than 0.1% (e.g. 0.3 or 0.15%) and lower than 1% (e.g.0.4%). A doping with 0.3% cobalt Co²⁺ ions was carried out in LMA offormula La_(0.9)Mg_(0.5)Al_(11.433)O₁₉, the material, after doping,having the formula: La_(0.9)Mg_(0.4985)Co_(0.0015)Al_(11.433)O₁₉. Thegeneral formula of the doped material is:La_(0.9)Mg_(0.5-x)Co_(x)Al_(11.433)O₁₉. It is also possible to startwith stoichiometric LMA (of formula LaMgAl₁₁O₁₉) and the formula afterdoping is LaMg_(1-x)Co^(x)Al₁₁O₉). A certain variation around thesedifferent compositions is possible.

The compound Sr_(1-x)Mg_(x)La_(y)Al_(12-y)O₁₉Sr_(1-x) La _(x) Mg _(x) Al_(12-x) O ₁₉ (or ASL) doped with cobalt, whereby in said compound, whichis a solid solution between SrAl₁₂O₁₉ and LaMgAl₁₁O₁₉ (LMA), the cobaltion replaces the magnesium ion (Mg) (formula of the doped material:Sr_(1-x)Mg_(x-z)Co_(z)La_(y)Al_(12-y)O₁₉ with o z x Sr_(1-x) La _(x) Mg_(x-y) Co _(y) Al _(12-x) O ₁₉ with 0<y<x).

The compound YAP, doped with cobalt (or YAlO₃:Co²⁺).

YAG doped with Co²⁺ of formula Y₃Al_(5-2x)Co_(x)Si_(x)O₁₂, the Si⁴⁺ ionmaking it possible to carry out a charge compensation.

Solid solutions based on YAG and doped with Co²⁺, said matrixes beinggarnets derived from YAG (Y₃Al₅O₁₂). The aluminium ion (Al) isprogressively replaced by gallium ions (Ga) or scandium ions (Sc)(Y₃Al_(5-x-y)Ga_(x)Sc_(y)O₁₂). Another possible solid solution isobtained by replacing yttrium by lutetium, which givesY_(3-x)Lu_(x)Al₅O₃, which can itself be doped with cobalt Co²⁺. Theformulas of these doped materials are respectivelyY₃Al_(5-x-y-2z)Ga_(x)Sc_(y)Co_(z)Si_(z)O₁₂ andY_(3-x)Lu_(x)Al_(5-2y)Co_(y)Si_(y)O₃.

In all these materials, the cobalt has a wide absorption band around 1.5μm. The width of this band and its wavelength position are dependent onthe matrix, but the band always remains close to 1.5 μm and is at leastbetween 1.5 and 1.6 μm.

In all these compounds, the Co²⁺ ion will be substituted with greater orlesser ease in the chosen matrix. With regards to LMA, said materialcontains Mg²⁺ ions, which can be substituted with cobalt ions withoutcharge compensation. The same situation arises for CaF₂, MgF₂, SrF₂ andBaF₂, as well as for ASL. The other compounds involve a chargecompensation.

All these compounds can also be deposited in thin film form, e.g. byliquid phase or molecular beam epitaxy or by the sol-gel method.

These compounds can also be obtained by Czochralski pulling (referencecan e.g. be made in this connection to the thesis of C. Borel given onNov. 12, 1990 at Paris VI University, pp 43-48) in solid form.

Molecular beam epitaxy is e.g. described in the thesis of E. Daran givenon Apr. 28, 1994 at INSA Toulouse, pp 57-60 and 64-76.

The sol-gel method is described in the document published in SPIE, vol.1794, Integrated Optical Circuits, p 303, 1992.

EP-653 824 gives detailed information on processes for producingsaturable absorber films by liquid phase epitaxy. For example, forobtaining a cobalt-doped film, it is possible to use a bath with thefollowing composition:

Pb/B : 12

Al/Y=4 (formation of the phase Y₃Al₅O₁₂)

Al/Co=10 to start, because the cobalt easily takes the place of thealuminium.

To obtain the value II (Co²⁺) of the cobalt, silicon in oxide form SiO₂is introduced into the bath. The silicon is there for compensating thecharge of the cobalt Co²⁺ by a Si⁴⁺ ion, both in substitution for 2aluminium Al³⁺ ions. The molar concentration of the silicon is the sameas that of the cobalt.

In thin film form (sol-gel film or epitaxied film), the thickness of thesaturable absorber films can be much less than in the case of solidsaturable absorbers. Thus, a few to about one hundred microns issufficient for obtaining a correct absorption coefficient for theoperation of the laser. Thus, due to said limited thickness, the overalldimensions of the microlaser remain small, particularly when the lasermedium serves as a substrate for the deposition of the film.

Optionally and as explained in the article by A. Eda et al, CLEO'92,paper CWG33, p 282 (Conf. on Laser and Electrooptics, Anaheim, U.S.A.,May 1992), it is possible to produce an array of microlenses in atransparent material (silica, etc.) on the surface of the lasermaterial. The typical dimensions of these microlenses are a diameter ofone hundred to a few hundred microns and radii of curvature of a fewhundred micrometers to a few millimeters. These microlenses are used forproducing “stable” cavities (the “plane-plane” cavity is not stable) ofthe plano-concave type. In the case of an optical pumping, they alsomake it possible to focus the pumping beam.

In order to produce a complete laser cavity, the active medium with itssaturable absorber layer or layers will be located between two mirrors6, 8. The input mirror, deposited by known processes, will preferably bea dichroic mirror having a maximum reflectivity (as close as possible to100%) at the wavelength of the laser and the highest possibletransmission (>80%) at the wavelength of the pump (generally about 980nm for Er-doped materials). The output mirror is also of the dichroictype, but permits the passage of a few per cent of the laser beam.

The advantage of such a structure is immediately clear, because at notime does it require an optical alignment of the different components.It also avoids problems linked with a structure, where the active mediumis codoped with active laser ions and saturable absorber ions. Thus,there is a separation of the active medium and the saturable absorbermedium and it is possible to regulate and choose independently thethicknesses and concentrations of the ions in said two media, hence agreater design freedom for the same.

The pumping of such a cavity is preferably an optical pumping. Thus, alaser diode at 980 nm is particularly suitable for the envisagedwavelength range.

The microlaser cavity can be placed in a mechanical box intended toreceive the laser pumping diode. It is also possible to have twoseparate boxes, one for the microlaser cavity and the other for pumpinglaser diode, both boxes being linked by an optical fibre, with the aidof a connector provided in each box. These structures are illustrated byEP-653 824.

The invention also relates to a process for the production of amicrolaser cavity, as described hereinbefore. This production can takeplace according to the following stages, all compatible with collectiveproduction. The process differs as a function of the saturable absorbertype. The saturable absorber can be in the form of a film deposited onthe laser material (case A), a solid material conditioned in thin stripform and bonded to the laser material (case B), or a mixture of the twotechnologies (case C): deposition of the film on a neutral substrate(e.g. silica) and assembly with the laser material. This substrate canthen be removed using known microelectronics processes (e.g. etching anintermediate lift-off layer).

The following stages can be distinguished in said process or processes:

1) The first stage consists of choosing the active laser material. Adescription has already been given of the different possible materials(YVO₄, YSO, YAG, LMA, etc.), as well as the different criteria enablingthe expert to choose among said different materials.

2) The second stage will be a stage of conditioning the chosen lasercrystal. It is oriented and cut into strips with a thickness between 0.5and 5 mm, and e.g. a diameter of 25 mm, optionally as a function of thecrystal axes for an anisotropic crystalline material. It is e.g.possible to use a diamond blade saw.

3) The third stage consists of grinding and polishing the strips and hastwo objectives:

on the one hand removing the surface, work hardening layer resultingfrom the cutting,

on the other hand, bringing the thickness of the strips to a levelslightly exceeding the specification of the microlaser, the activemedium thickness being a parameter conditioning certain microlasercharacteristics (width of the spectrum and number of longitudinalmodes). The ground strips close to the final thickness are polished onthe two faces with an optical quality.

4) A stage of preparing a saturable absorber.

As stated hereinbefore, several types of preparation can be carried out,corresponding to the different saturable absorber types.

4A. Deposition of Saturable Absorber Films (Structure of FIG. 1)

4A1) Chemical preparation of the material to be deposited (e.g.preparation of the colloidal solution forming the precursor of thesol-gel, using a process described in the article of Q. He et alentitled “Rare Earth Doped Aluminium Phosphate Sol-Gel Films” publishedin SPIE, vol. 1794, Integrated Optical Circuits II, 1992, p 303,preparation of the molten bath for epitaxy and for molecular beamepitaxy reference can be made to the aforementioned thesis of E. Daran(p 68).

4A2) Deposition of the material in film form, according to a known,specific process (e.g. deposition with a whirler of the colloidal,sol-gel precursor solution, liquid phase epitaxy of a saturable absorbermaterial and deposition by molecular jet epitaxy).

4A3) Repolishing the strips, with a view to removing any roughnesscaused by the deposition process and for bringing the thickness of thedeposited film to the desired thickness for the operation of themicrolaser (for a film said thickness is between 1 and 200 μm e.g. 10,50, 100, 150 μm).

4A4) Possible etching of spherical microsurfaces on the polished state,according to the aforementioned prior art in connection withmicrooptical components. This stage can optionally take place prior tothe deposition of the saturable absorber layer.

4A5) Deposition of dielectric input and output mirrors on plates, e.g.cold. These are preferably dichroic mirrors, obtained by a deposition ofdielectric multilayers.

Stage 4A4 can be performed before or after stages 4A1, 2 and 3, exceptin the case of liquid phase epitaxy, which takes place at a hightemperature and may destroy the microsurfaces. The micro-etching of thespherical surfaces (4A4) can be carried out before or after the filmdeposition stages.

4B. Solid Saturable Absorber

It then has the structure of FIG. 2, where references identical to thoseof FIG. 1 designate the same elements. The saturable absorber isdesignated by the reference 18.

4B1) Cutting the saturable absorber material, e.g. prepared byCzochralski pulling.

4B2) Polishing the two faces of the plates (planar, parallel faces).

4B3) Deposition of the input mirror on the laser material and the outputmirror on the saturable absorber, or vice versa, using methods describedin 4A5).

4B4) Assembly of a strip of laser material and a strip of saturableabsorber material, e.g. by bonding under a press (reference 20designating an adhesive film) or according to the process described inFrench patent application 96 08943 of Jul. 17, 1996 (assembly byintimate contact or molecular adhesion). The mirrors are outside theassembly.

4C) Mixed Technology

This case is illustrated in FIGS. 3 and 4. In the case of FIG. 3, thesaturable absorber 4 is previously produced on a mirror film 8 and evenon a substrate 26, by a film deposition method of the type describedhereinbefore. The substrate can be transparent at the emissionwavelength of the laser beam, in which case it does not have to beeliminated. The saturable absorber-mirror 8-substrate assembly is thenassembled with the active laser medium 2 by an adhesive coating 30.

In the case of FIG. 4, the saturable absorber 4 is produced on alift-off layer 24 to be etched and even formed on a substrate 28. Theassembly is connected to the amplifier medium 2 with the aid of anadhesive coating 22. The layer 24 is then etched, which makes itpossible to eliminate the substrate 28. The mirrors can then be producedin the aforementioned manner.

The process becomes common to the following stages:

5) Cutting individual microlaser chips with a surface of approximately 1mm² using a diamond circular saw (identical to that used inmicroelectronics).

6) Connection of the pumping diode to the microlaser.

As stated, it is also possible to mix these two processes. Use is madeof a transfer method such as a lift-off, which is well known in the art,in order to assemble the saturable absorber part with the laser materialand then remove the deposition substrate.

PERFORMANCE EXAMPLE

A LMA:Co crystal was produced and doped with 0.3% of Co²⁺ cobalt ions,which amounts to substituting for the Mg²⁺ ion in the formula of LMA(La_(0.9)Mg_(0.5)Al_(11.433)O₁₉). With cobalt doping said formulabecomes La_(0.9)Mg_(0.4985)Co_(0.0015)Al_(11.433)O₁₉.

The crystal is oriented <100>. This crystal was cut into small lamellasof diameter 5 mm. Different thicknesses were produced: 500, 750 and 1000μm. The two faces of the lamellas are polished, so as to obtain twofaces which are as planar and parallel as possible. These lamellas aremechanically assembled with a Yb—Er-doped glass lamella, which is thelaser material, and two mirrors forming the laser cavity. The mirrorsare produced by a multilayer dielectric treatment. They are designed tooperate at 1.5 μm, which is the wavelength used here. The glass:Er, Ybused is a material bought from Kigre, U.S.A. It has the following Er andYb composition: 0.6 to 0.8 wt. % erbium oxide and 20 wt. % ytterbiumoxide. A 0.5 or 0.75 or 1 mm thick glass lamella was used and polishedin order to obtain two planar, parallel faces.

The cavity is pumped by a laser diode emitting at around 980 nm (boughtfrom Spectra Diode Labs, U.S.A.).

The typical results obtained are:

repetition frequency : 1 to 50 kHz,

pulse duration : 5 to 20 ns,

mean power : 10 to 20 mW,

pulse energy : 5 to 10 μJ.

Another LMA:Co material doped with 0.15 instead of 0.3% made it possibleto obtain pulses with an even higher frequency (100 kHz and higher).

The production process described hereinbefore offers the possibility ofproducing microlasers on a large scale and therefore at low cost, whichis important for applications in fields such as cars. In addition, thethus produced microlaser has the already mentioned advantages, namelymonolithic, i.e. of flexible use, and requires neither optical setting,nor alignment, because the monolithic production process permits aself-alignment of the laser. Moreover, the microlasers according to theinvention make it possible to attain high repetition frequencies (200 Hzand higher).

Among the possible industrial applications of microlasers, reference canbe made to laser telemetry, laser micromachining and marking, laserinjection (for power lasers), the detection of pollutants, scientificand medical instrumentation, etc.

Moreover, the combination of microlasers and microoptical technologies(microlenses), whilst maintaining the advantage of collective andtherefore low cost production, makes it possible:

to improve the performance characteristics of the microlasers (stablecavities, focussing of the pump),

to produce optical Microsystems intended for particular applications,such as:

the production of 2D (optionally addressable) networks,

micro-lidar (remote sensing of wind speeds, pollution, etc.),

obstacle detection for cars,

laser telemetry,

compact, low cost, laser marking machines.

Several of these applications, particularly marking, micro-lidar,obstacle detection, telemetry, etc. require high peak powers andtherefore a switched operation. The microlaser according to theinvention lends itself well to such applications.

With a view to the production of a telemeter, the microlaser accordingto the invention can be combined with a device for measuring theduration of a time interval, as described e.g. in EP-706 100 or Frenchpatent application 96 02616 of Mar. 1, 1996. The device alsoincorporates means for receiving a light pulse reflected by an objectand for detecting the reception instant of said pulse, as well as meansfor detecting the emission instant of a pulse from the microlaser. Sucha telemeter can be installed on cars in order to avoid collisions orassist the driving of the car in the case of poor visibility.

We claim:
 1. Microlaser cavity incorporating: a solid active mediumemitting at least in a wavelength range between 1.5 and 1.6 μm and asaturable absorber of formula CaF₂:Co²⁺ or MgF₂:Co²⁺ or SrF₂:Co²⁺ orBaF₂:Co²⁺ or La_(0.9)Mg_(0.5-x)Co_(x)Al_(11.433)O₁₉ or YAlO₃:Co²⁺ orYAl_(5-2x)Co_(x)Si_(x)O₃YAl_((1-2x)) Co _(x) Si _(x) O ₃ orY₃Al_(5-x-y)Ga_(x)Sc_(y)O₁₂:Co²⁺ orY₃Al_(5-x-y-2z)Ga_(y)Sc_(z)Co_(z)Si_(z)O₁₂Y₃ Al _(5-x-y-2z) Ga _(x) Sc_(y) Co _(z) Si _(z) O ₁₂ or YLuAlO:Co²⁺ or Y_(3-x)Lu_(x)Al₅ ₁₂CoSiOY_(3-x) Lu _(x) Al ₅ O ₁₂ :Co ²⁺ or Y _(3-x) Lu _(x) Al _(5-2y) Co _(y)Si _(y) O ₁₂ or Sr_(1-x)Mg_(x)La_(y)Al_(12-y)O₁₂:Co²⁺ orSr_(1-x)Mg_(x-y)Co_(y)La_(z)Al_(12-z)O₁₂, (with o<y<x Sr_(1-x) La _(x)Mg _(x) Al _(12-x) O ₁₉ :Co ²⁺ or Sr _(1-x) La _(x) Mg _(x-y) Co _(y) Al_(12-x) O ₁₉, (with 0<y<x for the latter compound).
 2. Microlaser cavityaccording to claim 1, the saturable absorber being in the form of afilm.
 3. Microlaser cavity according to claim 2, the film having athickness between 1 and 150 μm.
 4. Microlaser cavity according to claim2 or 3, the film having been obtainable by the sol-gel method, or bymolecular beam or liquid phase epitaxy.
 5. Microlaser cavity accordingto claim 1, the solid active medium being constituted by a base materialchosen from among Y₃Al₅O₁₂ (YAG), LaMgAl₁₁O₁₉ (LMA), Y₂SiO₅ (YSO),GdVO₄, Y₃Sc₂Ga₃O₁₂ (YSGG), SrY₄(SiO₄)₃O (SYS), Ca₂Al₂SiO₇ (CAS) anddoped either with erbium or with chromium or with an erbium-ytterbiumcodoping, or an erbium-ytterbium-cerium codoping.
 6. Microlaser cavityaccording to claim 1, the solid active medium being a phosphate glassdoped with erbium and ytterbium, the erbium an ytterbium dopingoperations being respectively at between 0.5 and 0.9 and between 15 andwt. % oxide.
 7. Microlaser cavity according to claim 1, the cavity beingstable.
 8. Microlaser cavity according to claim 7, having an inputmirror and an output mirror, at least one of the two mirrors beingconcave.
 9. Microlaser cavity according to claim 1, also having amicrolens directly formed on the laser material.
 10. Process for theproduction of a microlaser cavity involving the production of asaturable absorber of formula CaF₂:Co²⁺ or MgF₂:Co²⁺ or SrF₂:Co²⁺ orBaF₂:Co²⁺ or La_(0.9)Mg_(0.5-x)Co_(x)Al_(11.433)O₁₉ or YAlO₃:Co²⁺ orYAl_(5-2x)Co_(x)Si_(x)O₃YAl_((1-2x)) Co _(x) Si _(x) O ₃ orY₃Al_(5-x-y)Ga_(x)Sc_(y)O₁₂:Co²⁺ orY₃Al_(5-x-y-2z)Ga_(x)Sc_(y)Co_(z)Si_(z)O₁₂ or Y_(3-x)Lu_(x)Al₅O₁₂:Co²⁺or Y_(3-x)Lu_(x)Al_(5-2y)Co_(y)Si_(y)O₁₂ orSr_(1-x)Mg_(x)La_(y)Al_(12-y)O₁₂:Co²⁺ orSr_(1-x)Mg_(x-y)Co_(y)La_(z)Al_(12-z)O₁₂ (with o<y<x Sr_(1-x) La _(x) Mg_(x) Al _(12-x) O ₁₉ :Co ²⁺ or Sr _(1-x) La _(x) Mg _(x-y) Co _(y) Al_(12-x) O ₁₉ , (with 0<y<x for the latter compound).
 11. Processaccording to claim 10, also involving the conditioning to apredetermined thickness of the constituent material of the solid activemedium.
 12. Process according to one of the claims 10 or 11, thesaturable absorber being produced in film form.
 13. Process according toclaim 12, the film having a thickness between 1 and 150 μm.
 14. Processaccording to claim 12, the saturable absorber being produced by epitaxyor the sol-gel method.
 15. Process according to claim 12, the saturableabsorber being produced from a solid material conditioned in thin stripform.
 16. Process according to claim 15, the thin strip being bonded tothe active laser material.
 17. Process according to claim 15, the thinsaturable absorber strip and the laser active medium being assembled bybonding, intimate contact or molecular adhesion.
 18. Process accordingto claim 12, the film being directly produced on the active lasermedium.
 19. Process according to claim 12, the film being previouslydeposited on a substrate, which is then assembled with the laser medium.20. Process according to claim 19, the substrate then being removed. 21.Laser telemetry device operating on the principle of the measurement ofthe travel time of a light pulse, characterized in that it comprises: apassively switched microlaser having a microlaser cavity according toclaim 1, means for receiving a light pulse reflected by an object anddetection of the reception time of said pulse, means for detection ofthe emission time of a pulse from the microlaser, a device for measuringthe time interval separating the emission time of a microlaser pulsefrom the reception time of a reflected beam.
 22. Car equipped with atelemeter according to claim
 21. 23. Process according to claim 10, thesolid active medium constituted by a base material being chosen fromamong Y₃ Al ₅ O ₁₂ (YAG), LaMgAl ₁₁ O ₁₉ (LMA), Y ₂ SiO ₅ (YSO), GdVO ₄, Y ₃ Sc ₂ Ga ₃ O ₁₂ (YSGG), SrY ₄(SiO ₄)₃ O (SYS), Ga ₂ Al ₂ SiO ₇(CAS) and doped either with erbium or with chromium or with anerbium-ytterbium codoping, or an erbium-ytterbium-cerium codoping.